Internal combustion engines as used heretofore have provisions for advancing the spark discharge in the engine cycle when the vehicle is cruising and for retarding the spark discharge when the vehicle is accelerating in response to intake manifold pressure. It has been found that emissions are reduced and engine performance and economy are improved if the spark is not retarded during "crowd" conditions, that is, during moderate vehicle acceleration. However, during full throttle acceleration the spark must be retarded to avoid engine knock.